1. Field of the Invention
This invention relates to a navigation assistance system for use on board an aircraft, comprising an inertial navigation module adapted for providing first aircraft positional information resulting at least partially from accelerometric and gyrometric measurements, and a satellite positioning module comprising at least one satellite navigation data receiver, said positioning module being adapted for determining second aircraft positional information from satellite positioning data received by said satellite navigation data receiver.
In particular, it is applicable to aiding in guidance of an aircraft during critical phase of flights, such as takeoff, final approach, or landing. Guidance of an aircraft during such phase of flights requires high precision aircraft localization information, but also a short time to alert (or TTA).
Time to alert is to designate the maximum time interval between receipt of signals providing degraded precision and reporting the degradation of such precision to the user. This time to alert must be as short as possible so that degradation of precision can be reported to the user as fast as possible.
2. Description of the Related Art
Guidance of an aircraft during approach and landing phases is generally done by means of an instrument landing system, or ILS. This radio navigation system is capable of providing high precision approaches. E.g., an ILS of category I (ILS Cat I) is capable of reaching decision levels of 200 feet (i.e., 61 m). However, such systems require multiple antennae and beacons to be available on the ground around the landing strip.
Such guidance can be replaced by guidance based on a satellite navigation system. Herein, a satellite navigation system is a system dedicated to navigation of the GNSS (Global Navigation Satellite System) type, such as GPS (Global Positioning System) or GLONASS.
In a known manner, such systems are based on a receiver receiving radiofrequency signals emitted by satellites of a satellite constellation, and the receiver determining from each signal received a distance between the receiver and the satellite transmitter of the signal so as to infer the position of the receiver therefrom.
Precision of such navigation systems is affected by several error types, in particular atmospheric errors, satellite timing and orbit errors, and errors due to multipath interference of the signals.
For correcting some of these errors, it is known to couple GPS or GLONASS satellite navigation systems to an SBAS-type space-based augmentation system. This is for instance EGNOS (Geostationary Navigation Overlay Service) or WAAS (Wide Area Augmentation System).
Associating the spatial augmentation system with GPS allows for more precise positional information (up to one meter horizontally and two meters vertically) to be obtained which is more reliable than that obtained by GPS only.
In particular, the spatial augmentation system has redundancies allowing for data degradation of GPS, GLONASS, or SBAS to be identified and reported to the user within six seconds.
Such an association allows for powerful so-called LPV (Localizer Performance with Vertical guidance) approaches to be obtained, with decision levels from 250 to 200 feet, i.e., approximately 76 to 61 meters, thus comparable to those of an ILS Cat I, without however requiring the installation of antennae and ILS beacons in close proximity to the landing strips.
The spatial augmentation system, coupled to GPS, provides both positional information and associated protection limits, warranting for horizontal and vertical precision given by the system. When precision of positional information decreases, protection limits will increase. This increase is reported to the user at a maximum of six seconds after degradation of precision occurred, this duration corresponding to the system's time to alert.
Moreover, when at least one of these protection limits exceeds a minimum precision predefined as an alert limit, a warning signal is emitted to the user.
Such a delay between the degradation of precision and reporting thereof to the user means that during this time of six seconds, the guiding system could broadcast erroneous aircraft positional information which is not compatible with the user's performance requirements, without the user being warned. However, in the course of final approach, takeoff, or landing guidance, such errors may lead to dangerous situations if detected belatedly.